85xx: Using proper I2C source clock divider for MPC8544

The MPC8544 RM incorrect shows the SEC_CFG bit in PORDEVSR2 as being
bit 26, instead it should be bit 28.  This caused in incorrect
interpretation of the i2c_clk which is the same as the SEC clk on
MPC8544.  The SEC clk is controlled by cfg_sec_freq that is reported
in PORDEVSR2.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>